This question / answer from the SVN FAQ may interest you:
How to completely delete a file from the repository history?
There are special cases where you might want to destroy all evidence of a file or commit. (Perhaps someone accidentally committed a confidential document.) This is not so simple because Subversion is intentionally designed to never lose information. Revised immutable trees that are different. Removing the revision from the story will cause a domino effect, creating chaos in all subsequent revisions and possibly invalidating all working copies.
The project has plans, however, someday it will implement a team svnadmin obliteratethat will complete the task of permanently deleting information. (See Question 516.)
Meanwhile, your only one in svnadmin dumpyour repository, then pass the dump file through svndumpfilter(excluding the bad path) to the command svnadmin load.
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